This invention relates to a printed circuit heating element. More specifically the present invention relates to a printed circuit heating element suitable for heating and welding two thermoplastic tubes together.
U.S. patent application Ser. No. 267,291 filed on June 4, 1981, now U.S. Pat. No. 4,369,779, discloses an apparatus for forming a sterile connection comprising a cutting means, means adapted to heat said cutting means, a pair of mounting blocks adapted to receive and hold two tubes to be joined, means to provide movement between said blocks and said cutting means to a position such that the cutting means is between said blocks and traversing where the blocks are adapted to receive tubes, means adapted to realign said blocks to a position where two different tube ends are aligned with and facing each other, and means to separate said blocks and said cutting means while urging said blocks together. The application discloses that the cutting means can take many forms but preferably is a laminate strip constructed of an etched stainless steel ribbon having on each side an acrylic adhesive layer, an aromatic polyimide layer, an acrylic adhesive layer and a copper ribbon.
During the use of the sterile docking apparatus of the aforesaid patent, it has been found that printed circuit cutting means made of the aforesaid laminate suffered breakdown of the adhesive in the inner adhesive layer and displayed insufficient mechanical strength, thereby resulting frequently in unsatisfactory welds, e.g., lack of strength. There exists a need for advantageous solutions to the problems of adhesive breakdown, insufficient mechanical strength and poor dissemination of heat to the outside of the cutting means.
Printed circuit heating elements are well known. Many of these elements involve either a metal substrate having a ceramic coating with a resistor layer adhered to the coating or an etched resistor laminated with layers of adhesive to insulation, which is often an aromatic polyimide resin, which is laminated with additional layers of adhesive to a metal cover.
U.S. Pat. No. 2,859,321, issued to Garaway on July 11, 1955, discloses an electrical resistance heating device in which printed heating elements are applied to one side of a metal base, the heating elements being insulated from the base by means of a devitrified ceramic enamel coating applied therebetween.
U.S. Pat. No. 3,922,386, issued to Ros on Nov. 25, 1975, discloses a heating element for use with printed circuits and which is itself a printed circuit comprising a filament consisting of a mixture of aluminum and aluminum oxide, the resistivity of the heating element formed of the filament being adjustable by varying the thickness of the filament. The filament is supported on a ceramic coating on a metal substrate.